The production of semiconductors into various types of integrated circuits, displays, memories, sensors and other devices typically requires cleaning of semiconductor surfaces to remove contaminants. In many cases the presence of even small amounts of contamination on a semiconductor surface will impair or destroy operation of devices formed from the contaminated semiconductor material.
A common semiconductor material used for production of semiconductor devices is silicon, which is most typically in the form of a silicon containing wafer. The so-called “RCA clean” has been the standard clean sequence utilized by the industry for cleaning silicon wafers for the past two decades. An RCA clean process is illustrated in FIG. 1, and comprises steps 1A-1I.
Referring to step 1A, one or more silicon-comprising semiconductor wafers are subjected to a so-called “piranha” cleaning step. The piranha cleaning step typically comprises dipping the wafers in an inorganic oxidant, such as a solution containing sulfuric acid and hydrogen peroxide. This step is intended to remove organic material from a surface of the semiconductor article. A typical piranha cleaning solution would comprise hydrogen peroxide (H2O2) and sulfuric acid (H2SO4) in a ratio of about 1:5 to about 1:50 (hydrogen peroxide:(H2SO4).
Referring to step 1B, the wafers are rinsed with deionized water. The deionized water rinse typically occurs at room temperatures, commonly from about 18° C. to about 23° C. Preferably, the rinse utilizes water with a high resistivity, such as from about 10 megohms-centimeter to about 18 megohms-centimeter.
Referring to step 1C, the wafers are subjected to a so-called “HF clean”. The HF clean is typically used to remove an oxide film from the surfaces of the semiconductor wafers. Such oxide film may be formed, for example, during the above-discussed piranha clean or due to exposure of the semiconductor wafer or other article to air or other sources of oxygen. The HF clean typically involves dipping the wafers in a solution of water and hydrofluoric acid, with the water:hydrogen fluoride ratio commonly being in the range of from approximately 1000:1 to approximately 100:1.
Referring to step 1D, the semiconductor wafers are rinsed with deionized water to remove hydrogen fluoride and various materials loosened from the surface of the wafer.
Referring to step 1E, the wafers are subjected to a so-called “Standard Clean 1” step, commonly referred to as an “SC1” step. The SC1 step is principally directed to removing various particulate materials from the semiconductor surfaces which can more easily attach as a result of the surface being made hydrophobic by the hydrogen fluoride cleaning step explained above, step 1C. In a typical SC1 step, the wafers are submerged in a solution of water, hydrogen peroxide and ammonium hydroxide (for example 5:1:1 by volume), at temperatures from about 75° C. to about 80° C. for a time of from about 2 minutes to about 15 minutes.
Referring to step 1F, the wafers are subjected to a deionized water rinse to remove the above-described SC1 solution from the wafers. Typically, this rinse, like the rinse in step 1D, is relatively short to minimize regrowth of oxide on the wafers.
Referring to step 1G, the wafers are subjected to a so-called “Standard Clean 2” step, commonly referred to as an “SC2” step. The SC2 step is thought to desorb atomic and ionic contaminants from the wafers. In particular, the SC2 step is intended to remove metals deposited on the wafer surface during the HF cleaning step and SC1 step. In a typical SC2 step, the wafers are submerged in a solution of H2O:HCI:H2O2 (for example 6:1:1 by volume). The SC2 step can be carried out at temperatures which are elevated above common room temperatures. Examples of elevated temperatures sometimes used are from about 75° C. to about 80° C. The SC2 step can be effected for various times, for example for times from about 1 to about 10 minutes.
Referring to step 1H, the wafers are subjected to a deionized water rinse, similar to the rinse described above regarding step 1D, to remove the above-described SC2 solution from the wafers.
Referring to step 1I, the wafers are subjected to a drying step. The drying step can be done using a variety of processes. Standard processes can include relatively more sophisticated isopropyl alcohol drying systems, spin rinse driers, and other drying technologies. One standard drying procedure employs a combined rinse and dry which involves: 1) loading a cassette of wafers into a rinse tank; 2) spraying the cassette with rinse water until the water reaches a predetermined level; and 3) quickly dumping the water. After several rinse/dump cycles, the wafers are typically dried by exposure to a heated nitrogen gas.
The above-described RCA clean has the disadvantage of requiring numerous discrete steps for wafer cleaning. This necessarily increases both equipment costs, costs of processing facility space, and costs for added labor and processing time to complete numerous processing steps. It has long been recognized that a standard cleaning process is needed for semiconductor articles which can provide both fewer discrete steps and the exceedingly high degree of cleanliness needed in the semiconductor processing industry.
The RCA clean also has the further disadvantage of requiring four distinct chemical based cleaning steps (steps 1A, 1C, 1F and 1H), each of which requires a different chemical bath solution. Accordingly, many types of equipment designed to perform the RCA clean require four distinct tanks of cleaning solutions: a tank for the piranha mixture, a tank for the hydrogen fluoride mixture, a tank for the SC1 mixture, and a tank for the SC2 mixture. This renders the equipment more expensive and bulky. There is also an ancillary requirement to dispose of spent chemicals. When more chemicals are used then there is associated increases in chemical disposal costs. Accordingly, it has long been recognized as strongly desirable to have improved cleaning processes which reduce the number and volume of chemicals needed to clean semiconductor articles.